It’s been said that luck is when preparation meets opportunity. Ever since I caught the AR-15 at-home-building bug a few years ago, I've thought several times that I would like to conduct a from-scratch, at-home build of the AR-15's beefier and more potent compatriot, the AR-10 chambered in .308 Win. In particular, I've wanted to pull together a flexible, jack-of-all-trades .308 Win. with a feature set that could comfortably transition between target shooting from a bench rest, use in the field and, if necessary, defensive applications.
With this build in mind, I've been looking at AR-10s with shorter 16"-ish barrels, more nature-friendly color schemes blending black and Flat Dark Earth (FDE) components, and feature sets leaning toward tactical applications. One manufacturer that's been on my radar for a couple of years now is Aero Precision. This company has a reputation for quality and offers an in-house version of the AR-10 called the M5E1. It is available as a factory complete rifle or the separate components for an at-home build can be purchased from the company's website.
I really liked what I learned about Aero Precision and the M5E1, so I dropped their information into the "flexible .308 project" pot simmering at the back of my mental stove in hopes that an opportunity to work with the company would present itself. It did in the form of a phone call from American Rifleman. Gil, what do you know about Aero Precision? Would you be interested in meeting with them to discuss a possible project? With a smile on my face I said that not only would I be interested, I already had a project in mind... . And the rest is history! Or, at least, the review you're reading now.
As with AR-15 at-home builds, investing some sweat equity into an AR-10 build makes it possible to incorporate the upgrades and features you want into the rifle while reducing the overall price of the gun when compared to a factory assembled model. Regardless of your AR building experience level, there are a few things to keep in mind when approaching an AR-10 project.
First, the component dimensions of the AR-10 are larger and therefore wholly incompatible with AR-15 platforms. That means that some AR-15 build tools, including pin punches and vice blocks, are not compatible and will need to be replaced with the correct sizes.
Secondly, AR-10 pattern rifles do not have the benefit of an agreed-upon set of technical standards like Mil-Spec AR-15s do. AR-10 parts sourced from different companies and vendors may not fit together properly. It's much simpler to work with a single component provider, like Aero Precision, with a catalog of wholly compatible parts.
All of the pieces for this build arrived together with the exception of the serial numbered lower receiver. That went to a local FFL so as to be transferred like any other firearm.
The foundation of this M5E1 rifle is an American-made Aero Precision M5 forged 7075-T6 aluminum upper and lower receiver set treated with Magpul's FDE Cerakote finish. The partially assembled flat top upper arrives with the ejection port cover and forward assist installed along with M4 feed ramps and a DPMS high profile (.210) tang. The lower features an integral trigger guard, screws for the bolt catch and take down pin detent hole, safety selector markings compatible with 45, 60 or 90 degree safety selectors and a gap-free fit for a variety of aftermarket pistol grips.
The Aero Precision 16" long 4150 chrome moly vanadium steel barrel is button rifled at a 1:10 twist rate with a QPQ corrosion resistant finish and a DPMS barrel extension. It's configured for a mid-length gas system and the muzzle is threaded at 5/8x24 T.P.I. for muzzle accessories. In this case, the muzzle was fitted with a distinctive VG6 Precision EPSILON 762 high performance muzzle brake. CNC machined from 17-4ph heat treated stainless steel and given a black nitride finish, this hybrid device reduces felt recoil and muzzle flash at the same time.
The gas system consisted of a phosphate finished Aero Precision .750 low profile gas block and the company's Melonited mid-length gas tube which ships with a gas block roll pin. Aero's light weight 15" FDE Cerakoted M5 Atlas R-One hand guard is machined from 6061-T6 aluminum with an integral optics rail and M-Lok accessory rails. The proprietary Atlas attachment system is both sturdy and easy to install. The back end of the hand guard aligns with the receiver, not the barrel nut. This means you don't have to waste time struggling to get the hand guard top rail to line up with the receiver rail. Slide into place, tighten down using the provided tools and you're ready to go.
The complete bolt carrier group incorporated into this build is Aero's black nitride finished .308 / 7.62 multi-caliber model machined from a billet of 8620 steel, milled with forward assist serrations and case hardened. The staked gas key is made from 4130 steel and attached using Grade 8 hardware while the firing pin is machined from 8740 steel with a hard chrome finish. The bolt is case hardened 9310 steel with all components shot peened and treated with a black nitride finish. The bolt assembly was paired with Radian's Raptor-LT AR10/SR25 ambidextrous charging handle. This handle is machined from 7075 aluminum, treated with a Mil-Spec Type III hard anodized finish and then over-molded with fiberglass reinforced polymer.
The lower receiver was fleshed out using Aero Precision's M5 MOE Lower Parts Kit which is available with either a black or FDE Magpul MOE pistol grip that has a hinged cover storage compartment. This parts kit does not include a trigger group for those who plan to build their rifles using an upgraded trigger.
In this case, a Geissele Automatics Super Semi-Automatic Enhanced (SSA-E) Trigger was used. To be honest, these triggers don't look like $240 triggers in the box, instead they look an awful lot like an ordinary Mil-Spec trigger. Where's the brightly colored aluminum block housing or the sexy slide-adjustable trigger shoe? But the story changes once the Geissele is installed and lubricated according to the instructions. The 3 lbs. 12 oz. trigger pull does not properly describe how good this trigger feels. Compared to a Mil-Spec trigger, it’s the difference between running a 100-yard dash in steel-toed work boots versus top of the line running shoes.
The shoulder stock assembly is Magpul's 8-position adjustable UBR GEN2 for AR10/SR25 platforms in FDE with buffer tube included. Designed to accommodate the increased recoil produced by big-bore ARs, the cheek piece for this stock is fixed in place in order to provide a consistent cheek weld regardless of the angled-toe rubber butt pad’s position. The customizable storage compartment can be reversed or the door can be completely removed to reduce the stocks weight. Slings can be attached via front and rear QD sling attachment points or the two footman's loops. Aero provided the 308 Carbine Buffer and 308 Carbine Buffer Spring that completed the stock assembly.
High-quality standards were clearly evident throughout all of the components included in this build and in the final fit and finish of the completed rifle. The Aero M5E1 tapped and twisted together smoothly without any mechanical issues. But as with every from-scratch build, there are a few things to keep in mind. Be careful opening the M5 lower parts kit so as to avoid losing the tiny 4-40 set screw for the threaded take down pin detent hole. This particular detent opening is located above the pistol grip, instead of next to the buffer tube like an AR-15. I couldn't get the timing of the muzzle device right using the provided crush washer so the device was installed using a set of shims instead.
The Geissele trigger group arrives with a set of pins and a tube lubricant with instructions as to how the lubricant should be applied. Following those instruction paid off at the range. The trigger and hammer pin were a tight fit in the receiver due to the Cerakote finish so they needed a few taps with a polymer headed gunsmithing hammer when being set in place. If your pins are tight, it's important to go slowly, tapping gently so as to avoid any possible damage to the receiver's finish. The Magpul UBR Gen 2 stock has a unique installation process with a few more steps than usual. Stick to the provided instructions and it will work out nicely.
In preparation for the range test, the freshly minted M5E1 was properly lubricated and outfitted with some top-notch accessories. Magazines used for informal and formal testing included the C Products Defense 20-round stainless steel model with an Isonite and proprietary black Teflon finish, Hexmag HX SR-25 FDE polymer 20-Round and Magpul Gen3 PMAG 10 LR/SR black polymer 10-round. All of them, like the rifle, functioned flawlessly throughout the range test.
The M5E1 rifle got warmed up on a cold winter's day at the range using a set of Ultradyne C4 Precision folding sights and an Accu-Tac BR-4 G2 quick-detach bipod. CNC machined here in the U.S. from 4140 chromoly steel and aerospace grade 7075 aluminum, the C4's Dynalign design provides a circle-within-circle sight picture similar to that used by Olympic target shooters for faster target acquisition. They provide for quick sighting adjustments and one-handed deployment in the field. The sight picture proved to be much more intuitive to work with than the typical square peg front sight post paired with the round hole aperture.
The ruggedly built Accu-Tac BR-4 bipod is milled from 6061 T6 aluminum and treated with a flat black hard coat anodized finish. The tool-less mounting system allows for side-to-side tilt adjustments along with a rock solid connection to the support rail. The rubber footed, 5-position legs rotate independently of each other with up to 2" of height adjustment suitable for bench tops or rough terrain. This bipod adds about a pound of weight to the rifle, which is not a bad thing when the goal is added stability.
Formal accuracy testing was conducted at 100 yards from a benchrest using a Bushnell Elite Tactical LRTS 4.5-18x44mm scope featuring the company's G3 Reticle and a FDE finish. The optic was mounted using Weaver Tactical 34 mm high, 6-hole rings. This rifle, optic and scope mount combination looked, and shot, like they were made for each other.
The Sig Sauer Elite Performance 175-gr. open tip match load printed a best 5-shot group of 0.93" with a five group average of 0.99". DoubleTap's Rifle Defense 125-gr. Nosler ballistic tip turned in a best group of 0.91" with an average of 0.97". The tightest groups of the test were produced using the Federal Premium Gold Medal Match 168-gr. Sierra Match King open tip match round. It yielded a best group of 0.63" with a group average of 0.72".
A bit of do-it-yourself sweat equity paired with an excellent M5E1 component set provided by Aero Precision paid off handsomely in both the banking and shooting departments. The rifle's components added up to just under $1800. Factory assembled AR-10s in this class, with stock features, start at around $2100 and quickly go up from there. The first .308 Win. chambered AR-10 that I shot half a decade ago had a retail price of $2,195 with shot group averages ranging from 1.17” to 1.36” in size. The Aero Precision kit was about $400 less and turned in group averages ranging from 0.72" to 0.99". That sounds like a worthwhile investment to me!